In recent years, increase in speed and image quality of an image forming apparatus, such as a laser printer or a copier, has been desired. Accordingly, the image forming apparatus typically employs a configuration, in which a toner image formed on an image bearing member, such as a photosensitive drum, is transferred onto an intermediate transfer member, such as a belt, and then the toner image is transferred from the intermediate transfer member onto a transferring material, such as a sheet.
A full-color machine of tandem type has been widely used as an image forming apparatus that can form a color image at a high speed with high image quality. For example, the full-color machine has a configuration in which four image forming portions of yellow (Y), magenta (M), cyan (C), and black (Bk) are arranged in parallel, and photosensitive drums of the image forming portions are in contact with an intermediate transfer belt that serves as an intermediate transfer member. Here, by using primary transfer members, the intermediate transfer belt is pressed to the photosensitive drums and hence comes into contact with the photosensitive drums.
With the image forming portions, toner images are superposed on the intermediate transfer belt, then the toner images are secondarily transferred onto a transferring material from the intermediate transfer belt, and the toner images are fixed by fixing. Thus, a full-color image is formed.
Meanwhile, the image forming apparatus that can form a full-color image has a full-color mode, in which the image forming portions of all colors are in operation while all the photosensitive drums are in contact with the intermediate transfer belt. In addition, the image forming apparatus may have a mono-color mode, in which at least one of the image forming portions (for example, black) is in operation while at least one of the photosensitive drums is separated from the intermediate transfer belt. Thus, the apparatus may have the two modes.
The apparatus has the two modes mainly to increase the life of the photosensitive drums of the image forming portions, and to decrease toner consumption. For example, in the mono-color mode in which only black is used, the photosensitive drums of the image forming portions out of operation are separated from the intermediate transfer belt and inhibited from contacting the intermediate transfer belt. Accordingly, wear of the surfaces of the photosensitive drums can be prevented, and the life of the photosensitive drums can be increased. If a blade is used for cleaning the photosensitive drum, since the rotation of the photosensitive drum is stopped, it is not necessary to use a toner for lubrication to prevent the blade to be curled. Thus, the toner consumption can be decreased.
However, in the mono-color mode, the number of the photosensitive drums being in contact with the intermediate transfer belt is smaller than that in the full-color mode. In particular, the number of support points to nip the intermediate transfer belt by the photosensitive drums and primary transfer members is small, and hence the nipping force for the intermediate transfer belt is small. As described above, in the mono-color mode in which the nipping force for the belt is small, a load that is exerted on the intermediate transfer belt may vary when a front edge of a sheet, which is a transferring material, enters a secondary transfer nip portion, which is defined by the intermediate transfer belt and the secondary transfer member, or when a rear edge of the sheet exits the secondary transfer nip portion. The variation in load may affect a driving member that drives the intermediate transfer belt, and may cause the speed of the intermediate transfer belt to vary. Consequently, density difference may occur in a toner image during the toner image is transferred at the primary transfer nip portion because of the variation in speed of the intermediate transfer belt.
Patent Literature 1 suggests a configuration in which, a pressing force of a primary transfer member to an intermediate transfer belt that is a primary transfer member, which comes into contact with an intermediate transfer belt, in a mono-color mode is larger than a pressing force in a full-color mode.
However, the number of support points for the intermediate transfer belt is not changed in the suggestion in Patent Literature 1. Hence, a certain pressing force of the primary transfer member is needed to increase a supporting force to a sufficient level. When the pressing force is excessively increased, the nip width between the primary transfer member and the intermediate transfer belt is excessively increased, likely resulting in occurrence of an image failure.
The present invention is made in light of the situations, and an object of the present invention is to provide an image forming apparatus that decreases variation in speed of an intermediate transfer belt, the variation which is generated when a transferring material exits a secondary transfer nip portion defined by a secondary transfer member and the intermediate transfer belt, and hence that can obtain an image with high image quality.